The present invention relates to an ink jet recording apparatus.
A type of ink jet recording apparatus known in the art discharges a plurality of ink droplets from the same nozzle of an ink jet head in one printing cycle so that a single ink dot is formed from these ink droplets. A recording apparatus of this type arranges a plurality of ink dots on recording paper so that the ink dots together form an image, etc., on the recording paper. The number of ink droplets to be discharged in one printing cycle is adjusted so as to adjust the gradation and the size of the dot, thereby realizing so-called xe2x80x9cmultiple gray level recordingxe2x80x9d.
However, when printing at a high speed, the carriage of the ink jet head is moved at a high velocity, whereby the landing positions of the ink droplets discharged from the same nozzle are likely to be shifted from one another in the carriage direction. Then, the ink dot formed from the ink droplets will have an oblong circular shape elongated in the carriage direction, thereby lowering the image quality.
In view of this, a method for enabling high-speed printing has been proposed in the art, in which two ink droplets are discharged from the same nozzle with the later discharged ink droplet being discharged with a higher discharging velocity than that of the previously discharged ink droplet so that the two ink droplets are allowed to merge in flight into a single ink droplet before landing, as disclosed in, for example, Japanese Laid-Open Patent Publication No. 59-133066.
In recent years, the density of an ink jet head has been increasing, whereby the dimensional error in an actuator, or other elements, the change over time in the characteristics of an actuator, etc., have an increasing influence on the ink droplet discharging velocity. Specifically, if an actuator, a pressure chamber, etc., has a dimensional error, or the like, the degree of deformation of the actuator or the behavior of ink in the pressure chamber in response to a driving signal will be different from those in a case where there is no dimensional error, or the like, for the same driving signal. Thus, if the dimensional error, or the like, varies among different actuators, the ink droplet discharging velocity will also vary among different nozzles. Such variations in the ink droplet discharging velocity lead to variations in the landing position, thereby resulting in deteriorations in the image quality such as a white streak in a solid print, for example. Particularly, with an ink jet head that discharges a plurality of ink droplets that are merged together in flight, such as that disclosed in the above publication, the error in the discharging velocity among different ink droplets is amplified, whereby variations in the landing position are likely to occur.
By matching the vibration period of an actuator with the natural vibration period thereof, it is possible to bring the actuator into resonance and to bring the ink meniscus vibration into resonance. In this way, it is possible to drive the actuator with a smaller amount of energy. Thus, it is possible to improve the discharging performance by effectively utilizing resonance.
Note that the term xe2x80x9cvibration period of an actuatorxe2x80x9d as used herein refers to the vibration period of the entire vibration system including the ink, etc., i.e., the vibration period of the actuator with the pressure chamber, etc., being filled with ink. Similarly, the term xe2x80x9cresonance frequency, or the like, of an actuatorxe2x80x9d as used herein refers to the resonance frequency, or the like, of the entire vibration system including the ink, etc.
However, even with an ink jet head utilizing resonance, if the resonance frequency varies among different actuators, the discharging velocity or the discharged ink volume will vary among different nozzles, thereby leading to a shift in the ink droplet landing position or variations in the ink dot size. Such variations in the landing position or the ink dot size will cause irregularities in the arrangement of ink dots on the recording paper, thereby lowering the image quality. Particularly, with an ink jet head that discharges a plurality of ink droplets that are merged together in flight, the discharging velocity or the discharged ink volume easily vary due to a shift in the resonance frequency. Effective countermeasures have not been taken in the prior art against such variations in the discharging velocity or the discharged ink volume due to a shift in the resonance frequency.
The present invention has been made in view of the above, and has an object to improve the recording quality by reducing the variations in the discharging velocity among nozzles.
Another object of the present invention is to provide an ink jet recording apparatus that utilizes resonance and discharges a plurality of ink droplets that are merged together in flight, in which the recording quality is improved by suppressing the variations in the discharging velocity or the discharged ink volume of a merged ink droplet.
An ink jet recording apparatus of the present invention includes: a head body provided with a plurality of nozzles and a plurality of pressure chambers, which are communicated to the respective nozzles and are filled with ink; a plurality of actuators provided in the head body each including a piezoelectric element and an electrode for applying a voltage across the piezoelectric element for applying a pressure on the ink in one of the pressure chambers so as to discharge ink from one of the nozzles; and a driving circuit for supplying a signal to the electrode of each actuator, wherein: the driving circuit applies, in one printing cycle, a driving signal composed of a plurality of pulse signals for discharging a plurality of ink droplets so that the ink droplets are merged together in flight; and the driving signal includes a pulse signal applied with an interval that is shorter than a predetermined pulse interval being equal to a Helmholtz period of a head, and a pulse signal applied with an interval that is longer than the predetermined pulse interval.
Note that the term xe2x80x9cHelmholtz period of a headxe2x80x9d as used herein refers to the natural period of the entire vibration system including the ink (an acoustic element), the actuator, etc.
Theoretically, the degree of resonance of the ink meniscus vibration increases as the pulse interval of a pulse signal is closer to the Helmholtz period. Therefore, the ink droplet discharging velocity increases as the pulse interval is closer to the Helmholtz period.
In this ink jet recording apparatus, the plurality of pulse signals to be applied in one printing cycle include a pulse signal applied with an interval that is shorter than the Helmholtz period, and a pulse signal applied with an interval that is longer than the Helmholtz period. Therefore, if the Helmholtz period shifts due to various factors such as a dimensional error in the actuator, the pressure chamber, etc., or a change in the characteristics of the actuator, the discharging velocity of one ink droplet may be increased by such a shift in the Helmholtz period while the discharging velocity of another ink droplet may be decreased by the shift. As a result, the shift component that increases the discharging velocity of the merged ink droplet (i.e., an ink droplet whose discharging velocity increases due to the shift in the Helmholtz period) and the shift component that decreases the discharging velocity of the merged ink droplet (i.e., an ink droplet whose discharging velocity decreases due to the shift in the Helmholtz period) are canceled out by each other to some degree, thereby suppressing the shift in the discharging velocity of the merged ink droplet. Thus, variations in the discharging velocity among different nozzles are reduced. Therefore, variations in the ink droplet landing position are reduced, thereby improving the recording quality.
Another ink jet recording apparatus of the present invention includes: a head body provided with a plurality of nozzles and a plurality of pressure chambers, which are communicated to the respective nozzles and are filled with ink; a plurality of actuators provided in the head body each including a piezoelectric element and an electrode for applying a voltage across the piezoelectric element for applying a pressure on the ink in one of the pressure chambers so as to discharge ink from one of the nozzles; and a driving circuit for supplying a signal to the electrode of each actuator, wherein: the driving circuit applies, in one printing cycle, a driving signal composed of a plurality of pulse signals for discharging a plurality of ink droplets so that the ink droplets are merged together in flight; and the driving signal includes a pulse signal applied with an interval that is shorter than a predetermined pulse interval that maximizes an ink droplet discharging velocity, and a pulse signal applied with an interval that is longer than the predetermined pulse interval.
Theoretically, a pulse interval that maximizes the ink droplet discharging velocity is the time interval that is equal to the Helmholtz period of the head, as described above. In practice, however, the discharging velocity may be maximized when the pulse interval is equal to a predetermined interval that is slightly shifted from the Helmholtz period. This may be due to various uncertainties such as the interference between adjacent actuators. Note however that such a predetermined pulse interval can be uniquely determined in advance by an experiment, etc.
In this ink jet recording apparatus, the plurality of pulse signals to be applied in one printing cycle include a pulse signal applied with an interval that is shorter than the predetermined pulse interval, and a pulse signal applied with an interval that is longer than the predetermined pulse interval. Thus, as in the previous ink jet recording apparatus described above, variations in the discharging velocity among different nozzles are suppressed, thereby improving the recording quality.
The plurality of pulse signals included in the driving signal may be applied in an order such that an absolute value of a difference between the pulse interval thereof and the predetermined pulse interval gradually decreases.
The driving signal may include a first pulse signal, a second pulse signal and a third pulse signal; and two of the first to third pulse signals may have pulse intervals that are shorter than the predetermined pulse interval, with the other pulse signal having a pulse interval that is longer than the predetermined pulse interval.
The driving signal may include a first pulse signal, a second pulse signal and a third pulse signal; and two of the first to third pulse signals may have pulse intervals that are longer than the predetermined pulse interval, with the other pulse signal having a pulse interval that is shorter than the predetermined pulse interval.
Still another ink jet recording apparatus of the present invention includes: a head body provided with a plurality of nozzles and a plurality of pressure chambers, which are communicated to the respective nozzles and are filled with ink; a plurality of actuators provided in the head body each including a piezoelectric element and an electrode for applying a voltage across the piezoelectric element for applying a pressure on the ink in one of the pressure chambers so as to discharge ink from one of the nozzles; and a driving circuit for supplying a signal to the electrode of each actuator, wherein: the driving circuit applies, in one printing cycle, a driving signal composed of a plurality of pulse signals for discharging a plurality of ink droplets so that the ink droplets are merged together in flight; and the driving signal includes a pulse signal having a pulse width that is shorter than a predetermined pulse width being equal to one half of a Helmholtz period of a head, and a pulse signal having a pulse width that is longer than the predetermined pulse width.
Theoretically, the degree of resonance of the ink meniscus vibration increases as the pulse width of a pulse signal is closer to a half period, i.e., one half of the Helmholtz period of the head. Therefore, the ink droplet discharging velocity increases as the pulse width of the pulse signal is closer to a predetermined pulse width that is equal to the half period.
In this ink jet recording apparatus, the plurality of pulse signals to be applied in one printing cycle include a pulse signal having a pulse width that is shorter than the predetermined pulse width, and a pulse signal having a pulse width that is longer than the predetermined pulse width. Therefore, even if the Helmholtz period shifts due to various factors such as a dimensional error in the actuator, the pressure chamber, etc., the shift component that decreases the ink droplet discharging velocity and the shift component that increases the ink droplet discharging velocity are canceled out by each other to some degree, thereby suppressing the shift in the discharging velocity of the merged ink droplet. Thus, variations in the discharging velocity among different nozzles are reduced, thereby improving the recording quality.
Still another ink jet recording apparatus of the present invention includes: a head body provided with a plurality of nozzles and a plurality of pressure chambers, which are communicated to the respective nozzles and are filled with ink; a plurality of actuators provided in the head body each including a piezoelectric element and an electrode for applying a voltage across the piezoelectric element for applying a pressure on the ink in one of the pressure chambers so as to discharge ink from one of the nozzles; and a driving circuit for supplying a signal to the electrode of each actuator, wherein: the driving circuit applies, in one printing cycle, a driving signal composed of a plurality of pulse signals for discharging a plurality of ink droplets so that the ink droplets are merged together in flight; and the driving signal includes a pulse signal having a pulse width that is shorter than a predetermined pulse width that maximizes an ink droplet discharging velocity, and a pulse signal having a pulse width that is longer than the predetermined pulse width.
Theoretically, a pulse width that maximizes the ink droplet discharging velocity is the pulse width that is equal to the half period of the Helmholtz period of the head, as described above. In practice, however, the discharging velocity may be maximized when the pulse width is equal to a predetermined pulse width that is slightly shifted from the half period. This may be due to various uncertainties such as the interference between adjacent actuators. Note however that such a predetermined pulse width can be uniquely determined in advance by an experiment, etc.
In this ink jet recording apparatus, the plurality of pulse signals to be applied in one printing cycle include a pulse signal having a pulse width that is shorter than the predetermined pulse width, and a pulse signal having a pulse width that is longer than the predetermined pulse width. Thus, as in the previous ink jet recording apparatus described above, variations in the discharging velocity among different nozzles are suppressed, thereby improving the recording quality.
The plurality of pulse signals included in the driving signal may be applied in an order such that an absolute value of a difference between the pulse width thereof and the predetermined pulse width gradually decreases.
The driving signal may include a first pulse signal, a second pulse signal and a third pulse signal; and two of the first to third pulse signals may have pulse widths that are shorter than the predetermined pulse width, with the other pulse signal having a pulse width that is longer than the predetermined pulse width.
The driving signal may include a first pulse signal, a second pulse signal and a third pulse signal; and two of the first to third pulse signals may have pulse widths that are longer than the predetermined pulse width, with the other pulse signal having a pulse width that is shorter than the predetermined pulse width.
A thickness of the piezoelectric element may be set to be 0.5 xcexcm to 5 xcexcm.
When the piezoelectric element has a reduced thickness, a dimensional error in the actuator, the pressure chamber, etc., are likely to have a significant influence on the discharging velocity, whereby the effect of reducing the variations in the discharging velocity among different nozzles is more pronounced.
Still another ink jet recording apparatus of the present invention includes: a head body provided with a plurality of nozzles and a plurality of pressure chambers, which are communicated to the respective nozzles and are filled with ink; a plurality of actuators provided in the head body each including a piezoelectric element and an electrode for applying a voltage across the piezoelectric element for applying a pressure on the ink in one of the pressure chambers so as to discharge ink from one of the nozzles; and a driving circuit for supplying a signal to the electrode of each actuator, wherein: the driving circuit applies, in one printing cycle, a driving signal composed of a plurality of pulse signals for bringing the actuator into resonance and discharging a plurality of ink droplets so that the ink droplets are merged together in flight; and a waveform generation frequency of the driving signal is set to be equal to a predetermined frequency at which a discharging velocity takes its peak value in an upwardly-protruding velocity curve in which the waveform generation frequency is a variable for a horizontal axis and a discharging velocity of a merged ink droplet is a variable for a vertical axis.
Note that in the present specification, the term xe2x80x9cresonance of an actuatorxe2x80x9d means the resonance of the actuator with the pressure chamber, etc., being filled with ink. Thus, the term means the resonance of the entire vibration system including the ink, the pressure chamber forming member, etc., but not the resonance of the actuator itself, which is not filled with ink. Note that such resonance of an actuator can be determined by, for example, measuring the displacement of the actuator or measuring the ink meniscus.
Moreover, the term xe2x80x9cwaveform generation frequencyxe2x80x9d is a variable that indicates the degree of expansion/contraction when all pulse signals of a driving signal are expanded/contracted uniformly in the time axis direction, and is specifically defined as follows. Where A [MHz] denotes a fundamental frequency (note that the fundamental frequency can be set arbitrarily), and B [xcexcs] denotes the total time of all the pulse signals of the driving signal of the fundamental frequency (specifically, the amount of time from the start of the potential transition of the first pulse to the end of the potential transition of the last pulse; hereinafter referred to as xe2x80x9cpulse total timexe2x80x9d), assume that the pulse total time is changed to C [xcexcs] as all the pulse signals of the driving signal are expanded/contracted uniformly in the time axis direction. Then, the variable f [MHz] expressed as f=Axc2x7B/C is defined as the waveform generation frequency. As is clear from the expression, the waveform generation frequency f decreases as the pulse total time C increases, and increases as the pulse total time C decreases. In other words, the waveform generation frequency f decreases as the pulse signals are expanded along the time axis, and increases as the pulse signals are contracted along the time axis.
Thus, when the waveform generation frequency is higher than the fundamental frequency, the pulse total time C of the driving signal is shorter than the pulse total time B of a driving signal at the fundamental frequency, i.e., the driving signal is a compressed driving signal. On the other hand, where the resonance frequency of a certain actuator is lower than the resonance frequency of a reference actuator, even if the same driving signal is input to all the actuators, the driving signal of the certain actuator appears to be more compressed than the driving signal of the reference actuator. Thus, it can be said that the waveform generation frequency and the resonance frequency of an actuator are inversely related to each other.
In this ink jet recording apparatus, the waveform generation frequency of the plurality of pulse signals of the driving signal is set to be equal to a predetermined frequency at which the discharging velocity of the merged ink droplet takes its peak value. In the vicinity of the peak value, the discharging velocity does not change substantially even if the waveform generation frequency somewhat shifts. Therefore, variations in the discharging velocity can be suppressed even if there occur variations in the resonance frequency among different actuators due to manufacturing errors of the head, etc. As a result, the recording quality can be improved.
Still another ink jet recording apparatus of the present invention includes: a head body provided with a plurality of nozzles and a plurality of pressure chambers, which are communicated to the respective nozzles and are filled with ink; a plurality of actuators provided in the head body each including a piezoelectric element and an electrode for applying a voltage across the piezoelectric element for applying a pressure on the ink in one of the pressure chambers so as to discharge ink from one of the nozzles; and a driving circuit for supplying a signal to the electrode of each actuator, wherein: the driving circuit applies, in one printing cycle, a driving signal composed of a plurality of pulse signals for bringing the actuator into resonance and discharging a plurality of ink droplets so that the ink droplets are merged together in flight; and a waveform generation frequency of the driving signal is set to be equal to a predetermined frequency at which a discharged ink volume takes its peak value in an upwardly-protruding discharged ink volume curve in which the waveform generation frequency is a variable for a horizontal axis and a discharged ink volume of a merged ink droplet is a variable for a vertical axis.
In this ink jet recording apparatus, the waveform generation frequency of the plurality of pulse signals of the driving signal is set to be equal to a predetermined frequency at which the discharged ink volume of a merged ink droplet takes its peak value. In the vicinity of the peak value, the discharged ink volume does not change substantially even if the waveform generation frequency somewhat shifts. Therefore, variations in the discharged ink volume can be suppressed even if there occur variations in the resonance frequency among different actuators due to manufacturing errors of the head, etc. As a result, the recording quality can be improved.
Still another ink jet recording apparatus of the present invention includes: a head body provided with a plurality of nozzles and a plurality of pressure chambers, which are communicated to the respective nozzles and are filled with ink; a plurality of actuators provided in the head body each including a piezoelectric element and an electrode for applying a voltage across the piezoelectric element for applying a pressure on the ink in one of the pressure chambers so as to discharge ink from one of the nozzles; and a driving circuit for supplying a signal to the electrode of each actuator, wherein: the driving circuit applies, in one printing cycle, a driving signal composed of a plurality of pulse signals for bringing the actuator into resonance and discharging a plurality of ink droplets so that the ink droplets are merged together in flight; and a waveform generation frequency of the driving signal is set to be greater than a predetermined frequency at which a discharged ink volume takes its peak value in an upwardly-protruding discharged ink volume curve in which the waveform generation frequency is a variable for a horizontal axis and a discharged ink volume of a merged ink droplet is a variable for a vertical axis.
In this ink jet recording apparatus, the waveform generation frequency of the plurality of pulse signals of the driving signal is set to be greater than a predetermined frequency at which the discharged ink volume of a merged ink droplet takes its peak value. When the waveform generation frequency is greater than the predetermined frequency, the discharged ink volume of the merged ink droplet is likely to be small. On the other hand, the waveform generation frequency being greater than the predetermined frequency is equivalent to the resonance frequency of the actuator being small. Accordingly, the amount of deformation of the actuator increases. As a result, the discharged ink volume of the merged ink droplet is likely to be large. Thus, if the waveform generation frequency is greater than the predetermined frequency, the change in the discharged ink volume due to the variations in the resonance frequency of the actuator and the change in the discharged ink volume due to the variations in the amount of deformation of the actuator are canceled out by each other. Therefore, variations in the discharged ink volume can be suppressed even if there occur variations among different actuators due to manufacturing errors of the head, etc. As a result, the recording quality can be improved.
Each pulse signal of the driving signal may have a potential decreasing waveform for depressurizing the pressure chamber, a potential holding waveform for holding a potential and a potential increasing waveform for pressurizing the pressure chamber so that an ink droplet is discharged when the pressure chamber is pressurized after it is depressurized; a potential falling time of the potential decreasing waveform of the pulse signal may be set to be less than or equal to a natural period of the actuator; and a potential holding time of the potential holding waveform of the pulse signal may be set to be less than or equal to xc2xd of the natural period of the actuator.
Note that the term xe2x80x9cnatural period of an actuatorxe2x80x9d as used herein refers to the natural period of the entire vibration system including the ink, etc. The natural period of the actuator is equal in meaning to the Helmholtz period of the head as described above.
Thus, the printing cycle is shortened, and the head response is improved.